Manchester code (biphase line encoding)
A self-clocking binary line code that encodes data as signal transitions; widely used for clock recovery and DC balance in systems such as Ethernet and RFID.
Overview
Manchester code is a binary line code that combines data and timing information by representing bits with defined transitions in the signal waveform. Because each bit interval contains at least one transition, receivers can recover a clock from the encoded stream without a separate clock line. The technique is sometimes described as a form of biphase or baseband phase shift keying, since information is conveyed by changes in phase rather than by steady voltage levels.
How it works
In a Manchester-encoded signal the logical value of a bit is indicated by the direction of the transition within the bit period. Different conventions exist for which transition corresponds to '1' or '0', so communicating systems must agree on the mapping. A related scheme, differential Manchester, encodes bits using the presence or absence of an extra transition at the start of the bit interval and always includes a transition at the middle.
Characteristics
- Self-clocking: frequent transitions permit receiver clock recovery.
- DC balance: equal time at high and low levels over each bit, reducing baseline wander.
- Robustness: missing or shifted transitions are easy to detect, aiding synchronization and simple error awareness.
- Bandwidth: requires higher bandwidth than non-return-to-zero (NRZ) coding because it introduces transitions within every bit period.
History and development
The term "Manchester" is associated with work in early digital communication and computing research; the name became common as the encoding was adopted in a variety of data links and networking standards. Over time it has been standardized in several forms and remains a pedagogical example of biphase line codes.
Uses and examples
Manchester encoding has been used in many practical systems where reliable clock recovery and DC balance are important. Notable examples include early Ethernet physical layers such as 10BASE-T and a range of RFID, magnetic and infrared signaling schemes. Industry standards that specify bit-level formats or physical layers often document which Manchester convention to use; for Ethernet see the relevant physical-layer specifications for details on implementation (Ethernet).
Practical considerations and variants
Designers choose Manchester or its variants when simplicity of clock recovery and immunity to baseline drift outweigh the extra bandwidth cost. Differential Manchester is often preferred where polarity ambiguity can occur because it relies on transitions rather than absolute levels. When higher spectral efficiency is needed, other encodings (for example NRZ with separate clock recovery techniques) are commonly used instead.
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Author
AlegsaOnline.com Manchester code (biphase line encoding) Leandro Alegsa
URL: https://en.alegsaonline.com/art/61195